Myostatin (or GDF-8) is a negative regulator of muscle growth and is structurally related to the transforming growth factor β (TGF-β) superfamily (McPherron et al 1997a). More particularly, myostatin is a potent negative regulator of skeletal muscle during development, and in adult life. Myostatin is also found in a wide range of species from fish to mammals and the myostatin protein is highly conserved and homologous across species (McPherron and Lee, 1997a). Myostatin exerts its biological effects through interaction with the cell surface receptor activin type IIB (Lee et al, 2001). Myostatin is also known to regulate its own expression via a mechanism that is incompletely understood at present (Spiller et al., 2002, Rebbapragada et al, 2003).
It has been demonstrated that myostatin inhibits myoblast proliferation and differentiation without inducing apoptosis or stimulating muscle protein breakdown (Thomas et al, 2000; Langley et al, 2002; Rios et al, 2001; Taylor et al, 2001). Knock-out mice for myostatin have greatly increased muscle mass over their entire body. Myostatin-null mice have approximately 30% greater body weight than normal mice, and exhibit a 2-3 fold increase in individual muscle weight due to muscle fibre hyperplasia and hypertrophy. Natural mutations in myostatin have been identified as being responsible for the “double-muscled” phenotype, such as the Belgian Blue and Piedmontese cattle breeds (McPherron et al, 1997b; Kambadur et al, 1997; Grobet et al, 1997). A similar phenotype has been observed in a human that has a defective myostatin gene (Schuelke et al, 2004). The interpretation of the role of myostatin in various biological processes via studies of myostatin null animals has been confounded by inability to distinguish between pre-natal developmental effects and effects that relate to the lack of myostatin during juvenile and adult life.
However, myostatin has been implicated in a number of disorders associated with muscle wasting, or muscle atrophy, such as that seen in individuals affected by HIV, cancer, prolonged bed rest, muscular dystrophy or in age related sarcopenia (Gonzalez-Cadavid et al, 1998; Langley et al, 2004; Zachwieja et al, 1999; Bogdanovich et al, 2002; WO2006/083183). It was demonstrated that in vivo administration of myostatin induces cachexia, a severe form of muscle wasting associated with cancer and sepsis (Zimmers et al, 2002) and that may also occur as a result of extended bed rest. Furthermore, up-regulation of myostatin in glucocorticoid-induced muscle atrophy has been observed (Ma et al, 2003). Changes in myostatin expression have been shown in other conditions, for example, up-regulated in cardiomyocytes after heart damage, and down regulated in regenerating muscle (Sharma et al, 1999).
Myostatin has also been linked with many other biological processes. For example, knockout transgenic mice have altered cortical bone structure indicating a role in osteogenesis (Hamrick 2003). Furthermore, myostatin has been shown to be involved in regulating glucose and fat metabolism, thus it may be implicated in type 2 diabetes and obesity (McPherron and Lee, 2002). Myostatin has also been shown to be involved in the inflammatory response during wound healing (WO2006/083182).
The key role played by myostatin in the regulation of muscle growth and differentiation and the pathology of many diseases and disorders has led to the search for antagonists of myostatin. Whilst many myostatin antagonists have been developed, such as anti-myostatin antibodies (U.S. Pat. No. 6,096,506 and U.S. Pat. No. 6,468,535); a truncated activin type IIB receptor, myostatin pro-domain and follistatin (WO 02/085306); myostatin inhibitors released into culture from cells overexpressing myostatin (WO 00/43781); dominant negatives of myostatin (WO 01/53350); and small peptides including the WMCPP domain which binds to and inhibits myostatin (US 2004/0181033); there are currently no myostatin antagonists in clinical use. Thus, there still exists a need to develop more potent myostatin antagonists for use as therapeutic agents.
Accordingly, it is an object of the invention to provide proteins with myostatin antagonist activity for the treatment of myostatin related disorders, and/or to provide the public with a useful choice.